1. Field of the Invention
This invention relates to an improved infrared detector assembly, and more particularly to one having an improved cryostat for generating fast cooldown of large infrared focal plane arrays.
2. Discussion
Infrared detection systems are often used in conjunction with munitions and night vision systems for sensing electro-magnetic radiation in the wavelength range of 4 to 16 micrometers. Because many such detection systems have infrared detector arrays, commonly referred to as focal plane arrays (FPA's), which are generally most sensitive when operated at cryogenic temperatures, a cooling system is required to produce and maintain the cryogenic operating temperatures. Furthermore, fluctuations in the temperature directly impact the sensitivity of the detectors and can result in undesirable electronic instability and electronic noise. Therefore, it is desirable to maintain the cooled detector at a constant temperature to eliminate the instability and noise during operation of the detection system.
Commonly, cryogenic cooling systems either take the form of a Joule-Thompson cryostat utilizing the conversion of pressurized gases to cryogenic liquids or a Stirling cryoengine with closed-cycle refrigeration. The cooling systems are used in conjunction with a vacuum insulated dewar in which the electromagnetic detector array is placed. The dewar is evacuated to remove thermally conductive gases which would otherwise occupy the area surrounding the detector array so that potential heat loss through convection and conduction is minimized. Furthermore, the evacuated dewar also prevents moisture from condensing on the detector array.
The dewar is cooled by placing an indented region ("coldwell") of the dewar in contact with an expansion chamber ("expander") of the cryogenic cooling system. Commonly, the expander has a cylindrical tube ("cold finger") having a cold end which supports a focal plane platform or "heat sink" upon which the detector array and related components are mounted. Alternately, the dewar can be constructed without a cold finger such that the detector array is mechanically supported directly by the focal plane platform. In either case, thermal energy is withdrawn from the detector array through the heat sink which is in thermally conductive communication with the cryogenic cooling system.
Since the cryogenic cooling system is in thermal communication with the heat sink, expansion of a high pressure gas (i.e. argon, nitrogen) within the coldwell causes thermal energy to be withdrawn from the detector array for permitting the electronic components to operate at their most effective temperature. In addition, it is desirable to simultaneously cool the coldshield, which surrounds the detector array, to reduce electronic noise and to improve the background scene.
In order to produce efficient conductive withdrawal of thermal energy from the electromagnetic detector array and the coldshield, the focal plane platform on which they are mounted must be fabricated from a material, or composition of materials, possessing specific metallurgical properties. Ideally these properties include high strength, a high modulus of elasticity and high thermal conductivity. Additionally, the focal plane platform must provide low thermal distortion characteristics to minimize premature detector failure.
A number of design constraints affect the design of the focal plane platform. Since the focal plane platform is a structural support member, it must have sufficient bending stiffness to minimize mechanical deflection of the electromagnetic detector. Such requirements become particularly significant when the infrared seeker assembly is used as part of munition system which is subjected to intense vibration and high levels of boost-phase acceleration. Another significant design parameter is the extent to which heat is transferred through the focal plane platform. More particularly, it is critical that the cooldown rate for the cold end components of the infrared detector assembly, mounted to the focal plane platform, be less than a predetermined maximum time requirement. Since infrared seekers are often used in expendable munitions which must identify a target soon after (or before) their launch, cooldown time is a primary design parameter.
In most modern applications, the use of large staring type hybridized focal plane arrays (FPA's) has necessitated the utilization of focal plane platforms or heat sinks having increased thickness. The thicker platforms promote superior hybrid reliability and reduces thermal fatigue related detector failures. Unfortunately, the utilization of a thicker focal plane platform increases the Joule load of the system which proportionately increases the cryostat cooling requirements. Currently, infrared detection systems using large area FPAs, silicon read-out chips, and larger coldshields are capable of producing a minimum cooldown time of about 10 to 12 seconds when equipped with a conventional Joule-Thompson cryostat. However, in most modern munitions applications, a cooldown time of about 5 seconds is specified as the maximum acceptable time requirement.